1. Technical Field
The present invention relates to an optical device and an optical modulation apparatus.
2. Related Art
A Mach-Zehnder optical device is known that uses a lead lanthanum titanium zirconium oxide (a PbLaZrTiO-type complex oxide, shortened hereinafter to “PLZT”) crystal, in which the change in refractive index with respect to applied electric field strength differs from that of LiNbO3 (LN), as shown in Patent Document 1, for example. In order to achieve high-speed operation with such an optical device, one known technique includes forming an insulating film of SiO2 on the PLZT crystal and then forming an electrode.    Patent Document 1: Japanese Patent Application Publication No. 2006-58837
However, when such an insulating film is formed between the electrode and the PLZT crystal, the distance between the electrodes and the PLZT crystal increases. As a result, the electric field generated according to the voltage applied to the electrode does not effectively reach the waveguide formed by the PLZT crystal. Accordingly, the strength of the electric field applied to the waveguide is decreased relative to the applied voltage. Therefore, when the Mach-Zehnder optical device is used as an optical modulator, the Vπ voltage, which is the applied voltage necessary to shift the phase of the input light by half a wavelength, has a high voltage value.